U.S. patent number 8,742,039 [Application Number 12/743,463] was granted by the patent office on 2014-06-03 for methods for making polyolefins.
This patent grant is currently assigned to Univation Technologies, LLC. The grantee listed for this patent is Mark B. Davis, Sun-Chueh Kao, Tae Hoon Kwalk. Invention is credited to Mark B. Davis, Sun-Chueh Kao, Tae Hoon Kwalk.
United States Patent |
8,742,039 |
Davis , et al. |
June 3, 2014 |
Methods for making polyolefins
Abstract
A method for making a polyolefin composition according to one
embodiment includes altering the concentration of the chain
transfer agent present in the reactor to control the HMW and LMW
fractions of the polyolefin composition.
Inventors: |
Davis; Mark B. (Lake Jackson,
TX), Kao; Sun-Chueh (Pearland, TX), Kwalk; Tae Hoon
(Belle Mead, NJ) |
Applicant: |
Name |
City |
State |
Country |
Type |
Davis; Mark B.
Kao; Sun-Chueh
Kwalk; Tae Hoon |
Lake Jackson
Pearland
Belle Mead |
TX
TX
NJ |
US
US
US |
|
|
Assignee: |
Univation Technologies, LLC
(Houston, TX)
|
Family
ID: |
40328561 |
Appl.
No.: |
12/743,463 |
Filed: |
November 19, 2008 |
PCT
Filed: |
November 19, 2008 |
PCT No.: |
PCT/US2008/012898 |
371(c)(1),(2),(4) Date: |
May 18, 2010 |
PCT
Pub. No.: |
WO2009/067201 |
PCT
Pub. Date: |
May 28, 2009 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20100249355 A1 |
Sep 30, 2010 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
61003792 |
Nov 20, 2007 |
|
|
|
|
Current U.S.
Class: |
526/113; 526/160;
525/240; 525/191; 526/943; 526/161 |
Current CPC
Class: |
C08F
10/00 (20130101); C08F 10/00 (20130101); C08F
2/38 (20130101); C08F 10/00 (20130101); C08F
2/00 (20130101); C08F 10/00 (20130101); C08F
4/65904 (20130101); C08F 4/6592 (20130101); C08F
4/659 (20130101); C08F 210/16 (20130101); C08F
4/65916 (20130101); C08F 4/65925 (20130101); C08F
2420/02 (20130101); C08F 210/16 (20130101); C08F
210/16 (20130101); C08F 2500/12 (20130101); C08F
2500/07 (20130101) |
Current International
Class: |
C08F
4/654 (20060101); C08F 4/6592 (20060101); C08F
4/64 (20060101); C08L 23/08 (20060101) |
Field of
Search: |
;526/113,160,161,943
;525/191,240 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
WO01/30862 |
|
May 2001 |
|
WO |
|
WO 2005/100414 |
|
Oct 2005 |
|
WO |
|
Primary Examiner: Lu; Caixia
Attorney, Agent or Firm: Schmidt; Jennifer A. Leavitt;
Kristina
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a National Stage entry under 35 U.S.C. 371 of
International Application No. PCT/US2008/012898, filed Nov. 19,
2008, and claims the benefit of Ser. No. 60/003,792, filed Nov. 20,
2007, the disclosure of which is incorporated by reference.
Claims
What is claimed is:
1. A method for making a polyolefin composition, the method
comprising: contacting one or more olefinic monomers in a single
reactor containing a bicomponent catalyst system comprising a first
catalyst component and a second catalyst component each having a
catalyst productivity and a chain transfer agent response; the
single reactor having a chain transfer agent concentration; the
chain transfer agent response of the second catalyst component
being different from that of the first catalyst component, wherein
the bicomponent catalyst system comprises at least one Group 15
containing metal compound; effectuating the polymerization of the
olefinic monomers to produce an olefin polymer having a melt flow
ratio; wherein the first catalyst component produces a high
molecular weight (HMW) fraction of the polymer, and the second
catalyst component produces a low molecular weight (LMW) fraction
of the polymer; altering the concentration of the chain transfer
agent present in the reactor to move peaks of the HMW and LMW
fractions of the polymer in a same direction, the altering also
causing the melt flow ratio of the polymer to change in a first
direction; adjusting at least one condition in the reactor to
achieve a change in molecular weight split wherein the change in
split causes the melt flow ratio of the polymer to reverse trend
and change in a second direction and wherein the change in split
maintains the flow index, I.sub.21, of the polymer at about a
target level, wherein flow index I.sub.21, is measured according to
ASTM 1238 (190.degree. C./21.6 kg).
2. The method of claim 1, wherein the chain transfer agent is
hydrogen or an aluminum alkyl.
3. The method as recited in claim 1, wherein the change in
molecular weight split is achieved by changing relative amounts of
the first and second catalyst components in the reactor.
4. The method of claim 1, wherein the altering and adjusting are
performed concurrently.
5. The method of claim 1, wherein the adjusting is performed by
changing relative amounts of the first and second catalyst
components in the reactor.
6. The method of claim 1, wherein the adjusting is performed by
selectively poisoning one of the catalyst components more than the
other catalyst component.
7. The method of claim 1, wherein the adjusting is performed by
selectively activating one of the catalyst components more than the
other catalyst component.
8. The method of claim 1, wherein the polymer is an ethylene
polymer.
9. The method of claim 1, wherein the first and second catalyst
components are present on a single support.
10. The method of claim 1, further comprising adding a third
catalyst component.
11. A method for making a polyolefin composition, the method
comprising: contacting one or more olefinic monomers in a single
reactor containing a bicomponent catalyst system comprising a first
catalyst component and a second catalyst component each having a
catalyst productivity and a chain transfer agent response; the
single reactor having a chain transfer agent concentration; the
chain transfer agent response of the second catalyst component
being different from that of the first catalyst component, wherein
the bicomponent catalyst system comprises at least one Group 15
containing metal compound; and effectuating the polymerization of
the olefinic monomers to produce an olefin polymer having a melt
flow ratio and a flow index; wherein the first catalyst component
produces a high molecular weight (HMW) fraction of the polymer, and
the second catalyst component produces a low molecular weight (LMW)
fraction of the polymer; reducing the concentration of the chain
transfer agent present in the reactor; and adjusting at least one
condition in the reactor to achieve a change in molecular weight
split wherein the change in split causes the melt flow ratio of the
polymer to reverse trend and change in a second direction while
maintaining about a constant flow index I.sub.21, wherein flow
index, I.sub.21, is measured according to ASTM 1238 (190.degree.
C./21.6 kg).
12. The method of claim 11, wherein the chain transfer agent is
hydrogen or an aluminum alkyl.
13. The method of claim 11, wherein the flow index of the polymer
is maintained by changing relative amounts of the first and second
catalyst components in the reactor.
14. The method of claim 11, wherein adjusting at least one
condition in the reactor to change the molecular weight split
comprises increasing the amount of the low molecular weight
fraction of the polymer.
15. The method of claim 11, wherein the adjusting is performed by
changing relative amounts of the first and second catalyst
components in the reactor.
16. The method of claim 11, wherein the adjusting is performed by
selectively poisoning one of the catalyst components more than the
other catalyst component.
17. The method of claim 11, wherein the adjusting is performed by
selectively activating one of the catalyst components more than the
other catalyst component.
18. The method of claim 11, wherein the polymer is an ethylene
polymer.
19. The method of claim 11, wherein the first and second catalyst
components are present on a single support.
20. The method of claim 11, further comprising adding a third
catalyst component.
Description
FIELD OF THE INVENTION
The present invention relates to polyolefin production, and more
particularly, the invention relates to controlling product
properties during the polymerization of polyolefins.
BACKGROUND
The term "bimodal" or "multimodal" as applied to polyolefin resins
usually means that the resin has at least two distinct ranges of
molecular weight that may impart desired properties to the product
in great variety. Bimodal resins were typically made in two
separate reactors connected in series, for example, a product
having a first molecular weight was moved directly from a first
reaction zone in which it was made and introduced to a second
reaction zone usually providing different polymerization conditions
for making a polymer composition. Two-stage processes are difficult
to control and, perhaps more important, have a capital
disadvantage. Moreover, frequently the polymer products are not
homogeneously mixed in that at least some particles are entirely of
one modality or the other. It is therefore desirable to find ways
of making homogeneous bimodal polyolefins in a single reactor.
Alternatively, one approach to making bimodal polyolefin
compositions in a single reactor has been to employ a mixed
catalyst system, in which one catalyst component makes a primarily
low molecular weight (LMW) product and the other catalyst component
produces a primarily high molecular weight (HMW). For example,
bimodal catalysts are often used to co-polymerize polymers having
two average molecular weights using a single catalyst system. By
including both of these catalyst components in the same catalyst
system, a bimodal product can be produced. The polymer having
different molecular weights are mixed at the molecular level
providing a polymer product that is relatively free of gels
compared to similar products made in staged-reactor or
series-reactor processes or by the blending of two distinct
unimodal resins.
Controlling the ratio of the components in the bimodal polymer
product or composition is a significant manufacturing concern.
Product properties of bimodal resins are often sensitive to
component split. For instance, in the manufacture of high-density,
high-molecular-weight film, to achieve the desired specification
may require control of component split within about 2% of the
setpoint.
The weight percentage or "split" of the HMW and LMW in the total
polymer product is greatly influenced by the relative amount of
each type of catalyst in the catalyst system. While theoretically,
a catalyst system containing proper amounts of each catalyst could
be generated and used to produce the desired split in a particular
case, in practice using such a system would be difficult, as the
relative productivities of the catalyst components can change with
variations in reactor conditions or poison levels.
A technique for changing the flow properties of a bimodal resin is
by changing the resin component split, or weight fraction of the
HMW component in the product. By modifying the relative amounts of
HMW and LMW components in the resin, flow properties can be changed
as well. Unfortunately, in some cases changing the split affects
more than one variable. In some products, changing the HMW split by
a few percent can significantly affect both resin flow index and
Melt Flow Ratio (MFR).
MFR is a ratio of two different melt flow index measurements, and
is used to quantify the shear-thinning of the polymer. As is well
known, melt flow index measurements measure the rate of extrusion
of thermoplastics through an orifice at a prescribed temperature
and load, and are often used as a means to discern molecular weight
of the overall polymer.
Generally, it has been believed in the art that reducing hydrogen
concentration during polymerization using a bimodal catalyst system
would increase product MFR by increasing the spread of the HMW and
LMW product components.
SUMMARY
The present invention is broadly directed to various systems and
methods for producing, and/or controlling properties of a
polyolefin product.
A method for making a polyolefin composition according to one
embodiment includes contacting one or more olefinic monomers in a
single reactor containing a bicomponent catalyst system comprising
a first catalyst component and a second catalyst component each
having a catalyst productivity and a chain transfer agent response;
the single reactor having a chain transfer agent concentration; the
chain transfer agent response of the second catalyst component
having a different sensitivity than that of the first catalyst
component; effectuating the polymerization of the olefinic monomers
to produce an olefin polymer having a melt flow ratio; wherein the
first catalyst component produces a high molecular weight (HMW)
fraction of the polymer, and the second catalyst component produces
a low molecular weight (LMW) fraction of the polymer; altering the
concentration of the chain transfer agent present in the reactor to
move peaks of the HMW and LMW fractions of the polymer in a same
direction, the altering also causing the melt flow ratio of the
polymer to change in a first direction; and adjusting at least one
condition in the reactor to achieve a molecular weight split where
the melt flow ratio change reverses direction.
A method for making a polyolefin composition according to another
embodiment includes contacting one or more olefinic monomers in a
single reactor containing a bicomponent catalyst system comprising
a first catalyst component and a second catalyst component each
having a catalyst productivity and a chain transfer agent response;
the single reactor having a chain transfer agent concentration; the
chain transfer agent response of the second catalyst component
having a different sensitivity than that of the first catalyst
component; and effectuating the polymerization of the olefinic
monomers to produce an olefin polymer having a melt flow ratio and
a flow index; wherein the first catalyst component produces a high
molecular weight (HMW) fraction of the polymer, and the second
catalyst component produces a low molecular weight (LMW) fraction
of the polymer; wherein reducing the concentration of the chain
transfer agent present in the reactor while maintaining about a
constant flow index causes the melt flow ratio to decrease.
A method for making a polyethylene composition suitable for blow
molding according to another embodiment includes contacting at
least ethylene in a single reactor with a bicomponent catalyst
system comprising a first catalyst component and a second catalyst
component each having a catalyst productivity and a hydrogen
response; the single reactor having a hydrogen concentration; the
chain transfer agent response of the second catalyst component
having a different sensitivity than that of the first catalyst
component; and effectuating the polymerization of the ethylene to
produce polyethyelene having a melt flow ratio and a flow index;
wherein the first catalyst component produces a high molecular
weight (HMW) fraction of the polyethylene, and the second catalyst
component produces a low molecular weight (LMW) fraction of the
polyethylene; wherein reducing the concentration of the hydrogen
present in the reactor while maintaining about a constant flow
index causes the melt flow ratio to decrease.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of the general methods,
systems and/or apparatus of certain embodiments of the
invention.
FIG. 2 is a schematic representation of the general methods,
systems and/or apparatus of certain embodiments of the
invention.
FIG. 3 is a schematic representation of the general methods,
systems and/or apparatus of certain embodiments of the
invention.
FIG. 4 is a schematic representation of the general methods,
systems and/or apparatus of certain embodiments of the invention
illustrating implementation in a gas phase polymerization reactor
system.
FIG. 5 is a schematic representation of the general methods,
systems and/or apparatus of certain embodiments of the invention
illustrating implementation in a gas phase polymerization reactor
system.
FIG. 6 is a schematic representation of the general methods,
systems and/or apparatus of certain embodiments of the invention
implementation in a liquid phase polymerization reactor system.
DETAILED DESCRIPTION
Before the present compounds, components, compositions, and/or
methods are disclosed and described, it is to be understood that
unless otherwise indicated this invention is not limited to
specific compounds, components, compositions, reactants, reaction
conditions, ligands, metallocene structures, or the like, as such
may vary, unless otherwise specified. It is also to be understood
that the terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting.
It must also be noted that, as used in the specification and the
appended claims, the singular forms "a," "an" and "the" include
plural referents unless otherwise specified. Thus, for example,
reference to "a leaving group" as in a moiety "substituted with a
leaving group" includes more than one leaving group, such that the
moiety may be substituted with two or more such groups. Similarly,
reference to "a halogen atom" as in a moiety "substituted with a
halogen atom" includes more than one halogen atom, such that the
moiety may be substituted with two or more halogen atoms, reference
to "a substituent" includes one or more substituents, reference to
"a ligand" includes one or more ligands, and the like. The
following description is made for the purpose of illustrating the
general principles of the present invention and is not meant to
limit the inventive concepts claimed herein. Further, particular
features described herein can be used in combination with other
described features in each of the various possible combinations and
permutations.
As used herein, all reference to the Periodic Table of the Elements
and groups thereof is to the NEW NOTATION published in HAWLEY'S
CONDENSED CHEMICAL DICTIONARY, Thirteenth Edition, John Wiley &
Sons, Inc., (1997) (reproduced there with permission from IUPAC),
unless otherwise noted.
The present invention is broadly directed to various systems and
methods for controlling properties of a multi-component polyolefin
product.
A general method 10 for making a polyolefin can be described, for
example, with reference to FIG. 1, in which, in step 12, one or
more olefinic monomers are contacted in a single reactor containing
a bicomponent catalyst system comprising a first catalyst component
and a second catalyst component each having a catalyst productivity
and a chain transfer agent response; the single reactor being
having a chain transfer agent concentration; the chain transfer
agent response of the second catalyst component having a different
sensitivity than that of the first catalyst component. In step 14,
the polymerization of the olefinic monomers is effectuated to
produce an olefin polymer having a melt flow ratio; wherein the
first catalyst component produces a high molecular weight (HMW)
fraction of the polymer, and the second catalyst component produces
a low molecular weight (LMW) fraction of the polymer. In step 16,
the concentration of the chain transfer agent present in the
reactor is altered to move peaks of the HMW and LMW fractions of
the polymer in a same direction, the altering also causing the melt
flow ratio of the polymer to change in a first direction. In step
18, at least one condition in the reactor is adjusted to achieve a
molecular weight split where the melt flow ratio change reverses
direction.
A general method 20 for making a polyolefin can be described, for
example, with reference to FIG. 2, in which, in step 22, one or
more olefinic monomers are contacted in a single reactor containing
a bicomponent catalyst system comprising a first catalyst component
and a second catalyst component each having a catalyst productivity
and a chain transfer agent response; the single reactor being
having a chain transfer agent concentration; the chain transfer
agent response of the second catalyst component having a different
sensitivity than that of the first catalyst component. In step 24,
the polymerization of the olefinic monomers is effectuated to
produce an olefin polymer having a melt flow ratio and a flow
index; wherein the first catalyst component produces a high
molecular weight (HMW) fraction of the polymer, and the second
catalyst component produces a low molecular weight (LMW) fraction
of the polymer. Reducing the concentration of the chain transfer
agent present in the reactor while maintaining about a constant
flow index causes the melt flow ratio to decrease.
A general method 30 for making a polyethylene suitable for blow
molding can be described, for example, with reference to FIG. 3, in
which, in step 32, at least ethylene is contacted in a single
reactor with a bicomponent catalyst system comprising a first
catalyst component and a second catalyst component each having a
catalyst productivity and a hydrogen response; the single reactor
being having a hydrogen concentration; the chain transfer agent
response of the second catalyst component having a different
sensitivity than that of the first catalyst component. In step 34,
the polymerization of the ethylene is effectuated to produce
polyethyelene having a melt flow ratio and a flow index; wherein
the first catalyst component produces a high molecular weight (HMW)
fraction of the polyethylene, and the second catalyst component
produces a low molecular weight (LMW) fraction of the polyethylene.
Reducing the concentration of the hydrogen present in the reactor
while maintaining about a constant flow index causes the melt flow
ratio to decrease.
Further details of making polyolefins, including specific
apparatuses adapted therefore, are described below, and each of the
below-described details are specifically considered in various
combination with these and other generally preferred approaches
described herein.
While the present invention is applicable to gas phase polyolefin
production, the broad concepts and teachings herein also have
applicability to many types of processes, including but not limited
to, gas phase, gas/solid phase, liquid/solid phase, gas/liquid
phase, and gas/liquid/solid phase catalyst reactor systems
including polymerized catalyst reactor systems; gas phase,
gas/solid phase, liquid/solid phase, gas/liquid phase, and
gas/liquid/solid phase batch charge preparation systems; etc.
For ease of understanding of the reader, as well as to place the
various embodiments of the invention in a context, much of the
following description shall be presented in terms of a commercial,
gas phase polyethylene production system. It should be kept in mind
that this is done by way of non-limiting example only.
Using the methods and systems as described herein results in
reliable, commercially useful and cost effective continuous
polyolefin production. Further, using the methodology presented
herein results in polymers with heretofore unavailable physical
properties.
The methods described herein may be useful in any reaction process,
including polymerization process, solution, slurry, and high
pressure processes. The invention in one aspect creates a polymer
suitable for blow molding, with heretofore unavailable
properties.
Polymerization Process
In each of the aforementioned generally preferred approaches and/or
embodiments, the polymers can be made in a variety of processes,
including but not limited to, gas phase, gas/solid phase,
liquid/solid phase, gas/liquid phase, and gas/liquid/solid phase
reactor systems including polymerization reactor systems; gas
phase, gas/solid phase, liquid/solid phase, gas/liquid phase, and
gas/liquid/solid phase mass transfer systems; gas phase, gas/solid
phase, liquid/solid phase, gas/liquid phase, and gas/liquid/solid
phase mixing systems; gas phase, gas/solid phase, liquid/solid
phase, gas/liquid phase, and gas/liquid/solid phase heating or
cooling systems; gas/solid phase and gas/solid/liquid phase drying
systems; etc.
Fluidized Bed Polymerization Reactor Systems
In each of the aforementioned generally preferred approaches and/or
embodiments, the reactor may form part of a fluidized bed
polymerization reactor system. Gas phase polymerization reactions
may be carried out in fluidized bed polymerization reactors, and
can also be formed in stirred or paddle-type reactor systems (e.g.,
stirred bed systems) which include solids in a gaseous environment.
While the following discussion will feature fluidized bed systems,
where the present invention has been found to be preferred and
especially advantageous, it is to be understood that the general
concepts relating to the use of continuity additives containing a
scavenger, which are discussed relevant to the preferred fluidized
bed systems, are also adaptable to the stirred or paddle-type
reactor systems as well.
A fluidized bed can generally include a bed of particles in which
the static friction between the particles is disrupted. In each of
the aforementioned generally preferred approaches and/or
embodiments, the fluidized bed system can be an open fluidized bed
system or a closed fluidized bed system. An open fluidized bed
system can comprise one or more fluids and one or more types of
fluidized solid particles and having one or more fluidized bed
surfaces that are exposed to an open uncontrolled atmosphere. For
example, an open fluidized bed system can be an open container such
as an open-top tank or an open well of a batch reactor or of a
parallel batch reactor (e.g., microtiter chamber). Alternatively,
the fluidized bed system can be a closed fluidized bed system. A
closed fluidized bed system can comprise one or more fluids and one
or more types of fluidized particles that are generally bounded by
a barrier so that the fluids and particles are constrained. For
example, a closed fluidized bed system may include a pipeline
(e.g., for particle transport); a recirculating fluidized bed
system, such as the fluidized bed polymerization reactor system of
FIG. 4; any of which may be associated with various residential,
commercial and/or industrial applications.
A closed fluidized bed system can be in fluid communication with an
open fluidized bed system. The fluid communication between a closed
fluidized bed system and an open fluidized bed system can be
isolatable, for example, using one or more valves. Such isolation
valves can be configured for unidirectional fluid flow, such as for
example, a pressure relief valve or a check valve. In general, the
fluidized bed system (whether open or closed) can be defined by
manufactured (e.g., man-made) boundaries comprising one or more
barriers. The one or more barriers defining manufactured boundaries
can generally be made from natural or non-natural materials. Also,
in general, the fluidized bed system (whether open or closed) can
be a flow system such as a continuous flow system or a
semi-continuous flow (e.g., intermittent-flow) system, a batch
system, or a semi-batch system (sometimes also referred to as a
semi-continuous system). In many instances, fluidized bed systems
that are flow systems are closed fluidized bed systems.
The fluidized bed in preferred embodiments is generally formed by
flow of a gaseous fluid in a direction opposite gravity. The
frictional drag of the gas on the solid particles overcomes the
force of gravity and suspends the particles in a fluidized state
referred to as a fluidized bed. To maintain a viable fluidized bed,
the superficial gas velocity through the bed must exceed the
minimum flow required for fluidization. Increasing the flow of the
fluidizing gas increases the amount of movement of the particles in
the bed, and can result in a beneficial or detrimental tumultuous
mixing of the particles. Decreasing the flow results in less drag
on the particles, ultimately leading to collapse of the bed.
Fluidized beds formed by gases flowing in directions other than
vertically include particles flowing horizontally through a pipe,
particles flowing downwardly e.g., through a downcomer, etc.
Fluidized beds can also be formed by vibrating or otherwise
agitating the particles. The vibration or agitation keeps the
particles in a fluidized state.
In very general terms, a conventional fluidized bed polymerization
process for producing resins and other types of polymers is
conducted by passing a gaseous stream containing one or more
monomers continuously through a fluidized bed reactor under
reactive conditions and in the presence of catalyst at a velocity
sufficient to maintain the bed of solid particles in a suspended
condition. A continuous cycle is employed where the cycling gas
stream, otherwise known as a recycle stream or fluidizing medium,
is heated in the reactor by the heat of polymerization. The hot
gaseous stream, also containing unreacted gaseous monomer, is
continuously withdrawn from the reactor, compressed, cooled and
recycled into the reactor. Product is withdrawn from the reactor
and make-up monomer is added to the system, e.g., into the recycle
stream or reactor, to replace the polymerized monomer. See for
example U.S. Pat. Nos. 4,543,399, 4,588,790, 5,028,670, 5,317,036,
5,352,749, 5,405,922, 5,436,304, 5,453,471, 5,462,999, 5,616,661,
5,668,228, and 6,689,847 all of which are fully incorporated herein
by reference. A basic, conventional fluidized bed system is
illustrated in FIG. 4. The reactor vessel 110 (also referred to
herein a "reactor") comprises a reaction zone 112 and a velocity
reduction zone 114. While a reactor configuration comprising a
generally cylindrical region beneath an expanded section is shown
in FIG. 4, alternative configurations such as a reactor
configuration comprising an entirely or partially tapered reactor
may also be utilized. In such configurations, the fluidized bed can
be located within a tapered reaction zone but below a region of
greater cross-sectional area which serves as the velocity reduction
zone of the more conventional reactor configuration shown in FIG.
4.
The reaction zone 112 includes a bed of growing polymer particles,
formed polymer particles and a minor amount of catalyst all
fluidized by the continuous flow of polymerizable and modifying
gaseous components, including inerts, in the form of make-up feed
and recycle fluid through the reaction zone. To maintain a viable
fluidized bed, the superficial gas velocity through the bed must
exceed the minimum flow required for fluidization which is
typically from about 0.2 to about 0.5 ft/sec. for polyolefins.
Preferably, the superficial gas velocity is at least 0.2 ft/sec
above the minimum flow for fluidization or from about 0.4 to about
0.7 ft/sec. Ordinarily, the superficial gas velocity will not
exceed 5.0 ft/sec and is usually no more than about 2.8 ft/sec.
On start-up, the reactor is generally charged with a bed of
particulate polymer particles before gas flow is initiated. Such
particles help to prevent the formation of localized "hot spots"
when catalyst feed is initiated. They may be the same as the
polymer to be formed or different. When different, they are
preferably withdrawn with the desired newly formed polymer
particles as the first product. Eventually, a fluidized bed
consisting of desired polymer particles supplants the start-up
bed.
Fluidization is achieved by a high rate of fluid recycle to and
through the bed, typically on the order of about 50 times the rate
of feed or make-up fluid. This high rate of recycle provides the
requisite superficial gas velocity necessary to maintain the
fluidized bed. The fluidized bed has the general appearance of
dense mass of individually moving particles as created by the
percolation of gas through the bed. The pressure drop through the
bed is equal to or slightly greater than the weight of the bed
divided by the cross-sectional area.
Referring again to FIG. 4, make-up fluids can be fed at point 119
via feed line 111 and recycle line 122. The composition of the
recycle stream is typically measured by a gas analyzer 121 and the
composition and amount of the make-up stream is then adjusted
accordingly to maintain an essentially steady state composition
within the reaction zone. The gas analyzer 121 can be positioned to
receive gas from a point between the velocity reduction zone 114
and heat exchanger 124, preferably, between compressor 130 and heat
exchanger 124.
To ensure complete fluidization, the recycle stream and, where
desired, at least part of the make-up stream can be returned
through recycle line 122 to the reactor, for example at inlet 126
below the bed. Preferably, there is a gas distributor plate 128
above the point of return to aid in fluidizing the bed uniformly
and to support the solid particles prior to start-up or when the
system is shut down. The stream passing upwardly through and out of
the bed helps remove the heat of reaction generated by the
exothermic polymerization reaction.
The portion of the gaseous stream flowing through the fluidized bed
which did not react in the bed becomes the recycle stream which
leaves the reaction zone 112 and passes into the velocity reduction
zone 114 above the bed where a major portion of the entrained
particles drop back onto the bed thereby reducing solid particle
carryover.
The recycle stream is then compressed in compressor 130 and passed
through heat exchanger 124 where the heat of reaction is removed
from the recycle stream before it is returned to the bed. Note that
the heat exchanger 124 can also be positioned before the compressor
130. An illustrative heat exchanger 124 is a shell and tube heat
exchanger, with the recycle gas traveling through the tubes.
The recycle stream exiting the heat exchange zone is then returned
to the reactor at its base 126 and thence to the fluidized bed
through gas distributor plate 128. A fluid flow deflector 132 is
preferably installed at the inlet to the reactor to prevent
contained polymer particles from settling out and agglomerating
into a solid mass and to maintain entrained or to re-entrain any
particles or liquid which may settle out or become
disentrained.
In this embodiment, polymer product is discharged from line 144.
Although not shown, it is desirable to separate any fluid from the
product and to return the fluid to the reactor vessel 110.
In accordance with an embodiment of the present invention, the
polymerization catalyst enters the reactor in solid or liquid form
at a point 142 through line 148. If one or more co-catalysts are to
be added, as is often the case, the one or more cocatalysts may be
introduced separately into the reaction zone where they will react
with the catalyst to form the catalytically active reaction product
and/or affect the reaction proceeding in the reactor system.
However the catalyst and cocatalyst(s) may be mixed prior to their
introduction into the reaction zone.
A continuity additive may be added in situ to the reactor system
100 via an appropriate mechanism such as feed line 148 or another
feed line 150.
The reactor shown in FIG. 4 is particularly useful for forming
polyolefins such as polyethylene, polypropylene, etc. Process
conditions, raw materials, catalysts, etc. for forming various
polyolefins and other reaction products are found in the references
incorporated herein. Illustrative process conditions for
polymerization reactions in general are listed below to provide
general guidance.
The reaction vessel, for example, has an inner diameter of at least
about 2 feet, and is generally greater than about 10 feet, and can
exceed 15 or 17 feet.
The reactor pressure in a gas phase process may vary from about 100
psig (690 kPa) to about 600 psig (4138 kPa), preferably in the
range of from about 200 psig (1379 kPa) to about 400 psig (2759
kPa), more preferably in the range of from about 250 psig (1724
kPa) to about 350 psig (2414 kPa).
The reactor temperature in a gas phase process may vary from about
30.degree. C. to about 120.degree. C. In one approach, the reactor
temperature is less than about 40.degree. C., 30.degree. C., more
preferably less than about 20.degree. C., and even more preferably
less than about 15.degree. C. below the melting point of the
polyolefin being produced. The process can run at even higher
temperatures, e.g., less than about 10.degree. C. or 5.degree. C.
below the melting point of the polyolefin being produced.
Polyethylene, for example, has a melting point in the range of
approximately 125.degree. C. to 130.degree. C.
The overall temperature in a gas phase process typically varies
from about 30.degree. C. to about 125.degree. C. In one approach,
the temperature at the point of highest temperature in the reactor
system is less than about 30.degree. C., more preferably less than
about 20.degree. C., and even more preferably less than about
15.degree. C. below the melting point of the polyolefin being
produced. In a system such as that shown in FIG. 4, the point of
highest temperature is typically at the outlet of the compressor
130.
Other gas phase processes contemplated include series or multistage
polymerization processes. Also gas phase processes contemplated by
the invention include those described in U.S. Pat. Nos. 5,627,242,
5,665,818 and 5,677,375, and European publications EP-A-0 794 200,
EP-B1-0 649 992, EP-A-0 802 202, and EP-B-634 421 all of which are
herein fully incorporated by reference.
In any of the embodiments described herein, the gas phase process
may be operated in a condensed mode, where an inert condensable
fluid is introduced to the process to increase the cooling capacity
of the reactor system. These inert condensable fluids are referred
to as induced condensing agents or ICA's. For further details of a
condensed mode processes see U.S. Pat. Nos. 5,342,749 and
5,436,304, which are herein fully incorporated by reference.
In an embodiment, the reactor utilized in embodiments of the
present invention is capable of producing greater than 500 lbs of
polymer per hour (227 Kg/hr) to about 300,000 lbs/hr (90,900 Kg/hr)
or higher of polymer, preferably greater than 1000 lbs/hr (455
Kg/hr), more preferably greater than 10,000 lbs/hr (4540 Kg/hr),
even more preferably greater than 25,000 lbs/hr (11,300 Kg/hr),
still more preferably greater than 35,000 lbs/hr (15,900 Kg/hr),
still even more preferably greater than 50,000 lbs/hr (22,700
Kg/hr) and most preferably greater than 65,000 lbs/hr (29,000
Kg/hr) to greater than 100,000 lbs/hr (45,500 Kg/hr).
Another illustrative fluidized bed polymerization reactor system
200 is shown in FIG. 5. As shown, the system 200 is a recirculating
system including a fast riser 202, a downcomer 204, and a
recirculating pump 206. The monomer(s) and catalyst are added to
recycle line 208 via feed 210. In this type of system, the
polymerization product is formed primarily in the fast riser 202,
but continues to form throughout the system. Polymer particles
formed in the fast riser 202 pass through line 212 to an upper
inlet port 214 of the downcomer 204. The polymer particles gather
in the downcomer, where they move downwardly in a dense, slow
moving bed. The bed formed in the downcomer can be considered a
fluidized bed. Particulate polymer product is discharged from line
216. Although not shown, it is desirable to separate any fluid from
the product and to return the fluid to the reactor system 200.
Other Reactor Systems
Slower moving masses of particles, while considered "fluidized" for
purposes of embodiments of the invention, are also referred to in
the art as "moving beds." Moving beds include particles in such
things as mass flow bins, downcomers, etc. where solids are slowly
moving through a vessel.
Stirred bed system, while considered "fluidized" for purposes of
embodiments of the invention, include beds stirred or otherwise
agitated by a member such as a paddle or plunger rotating or moving
through the bed (e.g., stirred bed reactor, blender, etc.). Other
types of stirred bed systems can be formed by a rotating drum
(e.g., with or without internal baffles to enhance mixing), a
vessel moving in a see-saw manner, agitation including ultrasonic
vibrations applied to the particles or their container, etc.
Liquid Phase Reactor Systems
In each of the aforementioned generally preferred approaches and/or
embodiments, the reactor may form part of a liquid phase reactor
system. Referring to FIG. 6, a liquid phase polymerization system
300, such as a slurry, suspension or solution reactor system,
according to one approach generally comprises a reactor vessel 302
to which an olefin monomer and a catalyst composition are added,
such as via feed lines 304 and 306, respectively, or as a mixture
combined prior to addition to the reactor vessel 302. Additional
materials can be fed to the reactor vessel 302 via feed lines 304,
306, or an additional feed line or lines. The reactor vessel 302
typically contains a liquid reaction medium for dissolving and/or
suspending the polyolefin. The liquid reaction medium may consist
of the bulk liquid monomer or an inert liquid hydrocarbon that is
nonreactive under the polymerization conditions employed. Although
such an inert liquid hydrocarbon need not function as a solvent for
the catalyst composition or the polymer obtained by the process, it
usually serves as solvent for the monomers employed in the
polymerization. Among the inert liquid hydrocarbons suitable for
this purpose are isopentane, hexane, cyclohexane, heptane, benzene,
toluene, and the like. Slurry or solution polymerization systems
may utilize subatmospheric or superatmospheric pressures and
temperatures in the range of about 40.degree. C. to about
300.degree. C. A useful liquid phase polymerization system is
described in U.S. Pat. No. 3,324,095, which is herein incorporated
by reference.
Reactive contact between the olefin monomer and the catalyst
composition may be maintained by constant stirring or agitation,
e.g., by a member such as a paddle 308 or plunger rotating or
moving through the reactor vessel 302 (e.g., stirred reactor,
blender, etc.). Other types of liquid phase polymerization systems
can be formed by a rotating drum (e.g., with or without internal
baffles to enhance mixing), a vessel moving in a see-saw manner,
agitation including ultrasonic vibrations applied to the materials
or vessel, etc.
The reaction medium containing the olefin polymer product and
unreacted olefin monomer is withdrawn from the reactor vessel 302
continuously via outlet line 310. The olefin polymer product is
separated by separator 312, and moved from the system via line 314.
The unreacted olefin monomer and liquid reaction medium are
recycled into the reactor vessel 302 via recycle line 316.
Polymer Products
The term "polymer" as used herein refers to a macromolecular
compound prepared by polymerizing monomers of the same or a
different type. A polymer refers to homopolymers, copolymers,
terpolymers, interpolymers, and so on. The term "interpolymer" used
herein refers to polymers prepared by the polymerization of at
least two types of monomers or comonomers. It includes, but is not
limited to, copolymers (which usually refers to polymers prepared
from two different monomers or comonomers), terpolymers (which
usually refers to polymers prepared from three different types of
monomers or comonomers), and tetrapolymers (which usually refers to
polymers prepared from four different types of monomers or
comonomers), and the like. The term "monomer" or "comonomer" refers
to any compound with a polymerizable moiety which is added to a
reactor in order to produce a polymer. The term "polyolefin" refers
to any polymer containing an olefinic monomer.
In each of the aforementioned generally preferred approaches and/or
embodiments, the polymers may be produced from monomers selected
from ethylene, propylene, 1-butene, 1-hexene, 1-pentene,
4-methyl-1-pentene, 1-octene, 1-decene, vinyl-cyclohexene, styrene,
ethylidene norbornene, norbornadiene, 1,3-butadiene, 1,5-hexadiene,
1,7-octadiene, 1,9-decadiene, or a combination thereof. The
polymers may be homopolymers of ethylene or copolymers of ethylene
with one or more C.sub.3-C.sub.20 alpha-olefins. Thus, copolymers
having two monomeric units are possible as well as terpolymers
having three monomeric units. Particular examples of such polymers
include ethylene/1-butene copolymers, ethylene/1-hexene copolymers,
ethylene/1-octene copolymers, ethylene/4-methyl-1-pentene
copolymers, ethylene/1-butene/1-hexene terpolymers,
ethylene/propylene/1-hexene terpolymers and
ethylene/propylene/1-butene terpolymers. When propylene is employed
as a comonomer, the resulting linear low density polyethylene
copolymer preferably has at least one other alpha-olefin comonomer
having at least four carbon atoms in an amount of at least 1
percent by weight of the polymer. Accordingly, ethylene/propylene
copolymers are possible.
Polymerization conditions generally refer to temperature, pressure,
monomer content (including comonomer concentration), catalyst
concentration, cocatalyst concentration, activator concentration,
etc., that influence the molecular weight of the polymer produced.
The weight-average molecular weight (M.sub.w) of a homopolymer,
copolymer, or other interpolymer can be measured by gel permeation
chromatography as described in U.S. Pat. No. 5,272,236, which is
incorporated by reference herein in its entirety. For ethylene
polymers or interpolymers, one method to determine the molecular
weight is to measure the melt index according to ASTM D-1238
Condition 190.degree. C./2.16 kg (formerly known as "Condition E"
and also known as "12"). Generally, melt index (I.sub.2) is
inversely related to the molecular weight of an ethylene polymer.
The higher the molecular weight, the lower the melt index
(I.sub.2), although the relationship is not necessarily linear.
Another measurement used in characterizing the molecular weight of
ethylene polymers involves measuring the melt index with a higher
weight in accordance with ASTM D-1238, Condition 190.degree.
C./21.6 kg (formerly known as "Condition F" and also known as
"121"). Melt Flow Ratio (MFR) is defined herein as the ratio of the
flow index (FI or I.sub.21) divided by the melt index (I.sub.2),
i.e., I.sub.21/I.sub.2. Molecular weight distribution is the weight
average molecular weight (M.sub.w) divided by number average
molecular weight (Mn), i.e., M.sub.w/Mn.
In one preferred approach, the polymer is suitable for blow molding
applications. Generally, high performance blow molding resins have
a bimodal molecular weight distribution. This means that the resin
comprises at least two polymer components, one of the at least two
components having a higher average molecular weight (sometimes
referred to as the "HMW polymer component") than another of the at
least two components (sometimes referred to as the "LMW polymer
component").
In one particularly preferred approach, polyethylene suitable for
blow molding is produced. The properties of such polyethylene may
include a flow index range between about 10 and about 50 dg/min,
more preferably between about 20 and about 40 dg/min. The MFR of
the resin may vary from greater than about 250 to less than about
100 (I.sub.21/I.sub.2), preferably between about 250 and 100
(I.sub.21/I.sub.2), more preferably centered around 150
(I.sub.21/I.sub.2). The density of such polyethylene resins may be
less than about 1 g/cc, preferably between about 0.9 and about 1.0
g/cc, more preferably between about 0.955 and about 0.960 g/cc.
In general, for example, the reactor systems and methods described
herein can be used in connection with liquids and/or gases having a
wide range of fluid properties, such as a wide range of
viscosities, densities and/or dielectric constants (each such
property being considered independently or collectively as to two
or more thereof). For example, liquid fluids can generally have
viscosities ranging from about 0.1 cP to about 100,000 cP, and/or
can have densities ranging from about 0.0005 g/cc to about 20 g/cc
and/or can have a dielectric constant ranging from about 1 to about
100. In many embodiments of the invention, the bulk material is a
gaseous fluid. Gaseous fluids can, for example, generally have
viscosities ranging from about 0.001 to about 0.1 cP, and/or can
have densities ranging from about 0.0005 to about 0.1 g/cc and/or
can have a dielectric constant ranging from about 1 to about
1.1.
The bulk material can include relatively pure gaseous elements
(e.g., gaseous N.sub.2, gaseous H.sub.2, gaseous O.sub.2). Other
components can include relatively pure liquid, solid, or gaseous
compounds (e.g., liquid or solid catalyst, gaseous monomer, air).
The various systems of embodiments of the invention can also
include single-phase or multi-phase mixtures of gases, solids
and/or liquids, including for example: two-phase mixtures of solids
and gases (e.g., fluidized bed systems), mixtures of gasses with a
single type of particle, mixtures of gasses with different types of
particles (e.g., polymer and catalyst particles); and/or
three-phase mixtures of gasses, liquids and solids (e.g., fluidized
bed with liquid catalyst being added). Particular examples of
preferred fluids are described herein, including in discussion
below regarding preferred applications of the methods and devices
of embodiments of the invention.
Catalyst Systems
In each of the aforementioned generally preferred approaches and/or
embodiments, a bicomponent catalyst system is used. The term
"bicomponent catalyst system" as used herein means catalyst systems
having at least two catalyst components, and may indeed include
catalyst systems including several different catalyst
components.
For example, the bicomponent catalysts system may include at least
one Group 15 containing metal compound such as
bis(2,3,4,5,6-pentamethylphenyl amido ethyl)anine zirconium
dibenzyl and at least one metallocene compound such as
bis(n-butylcyclopentadienyl)zirconium dichloride or
tetramethylcyclopentadienyl)(n-propylcyclopentadienyl)zirconium
dichloride.
In one approach, bicomponent catalyst systems may include catalyst
systems where differing catalysts are present on a single
support.
In another approach, bicomponent catalyst systems may include
systems where catalysts are not on a single support. Such catalyst
systems may include mixtures of catalysts in a common carrier, as
well as catalysts independently fed to the reactor system.
In a further approach, one or more catalysts are employed along
with a catalyst system having differing catalysts present on a
single support.
For simplicity, much of the present description will refer to a
catalyst system containing two catalyst components. However, it
should be kept in mind that the teachings herein extend to
embodiments where the bicomponent catalyst system includes more
than two catalyst components.
In some embodiments, the first catalyst is a high molecular weight
catalyst and the second catalyst is a low molecular weight
catalyst. Alternatively, the first catalyst is a low molecular
weight catalyst and the second catalyst is a high molecular weight
catalyst.
A high molecular weight catalyst and a low molecular weight
catalyst are determined with reference to each other. One does not
know whether a catalyst is a high molecular weight catalyst or a
low molecular weight catalyst until after another catalyst is also
selected. Therefore, the terms "high molecular weight" and "low
molecular weight" used herein when referring to a catalyst are
merely relative terms and do not encompass any absolute value with
respect to the molecular weight of a polymer. After a pair of
catalysts are selected, one can easily ascertain which one is the
high molecular weight catalyst by the following procedure: 1)
select at least one monomer which can be polymerized by the chosen
catalysts; 2) make a polymer from the selected monomer(s) in a
single reactor containing one of the selected catalysts under
pre-selected polymerization conditions; 3) make another polymer
from the same monomer(s) in a single reactor containing the other
catalyst under substantially the same polymerization conditions;
and 4) measure the melt index I.sub.2 for the respective
interpolymers. The catalyst that yields a lower I.sub.2 is the
higher molecular weight catalyst. Conversely, the catalyst that
yields a high I.sub.2 is the lower molecular weight catalyst. Using
this methodology, it is possible to rank a plurality of catalysts
based on the molecular weight of the polymers they can produce
under substantially the same conditions. As such, one can select
three, four, five, six, or more catalysts according to their
molecular weight capability and use these catalysts simultaneously
in a single polymerization reactor to produce polymers with
tailored structures and properties.
In some embodiments, the high molecular weight catalysts and the
low molecular weight catalysts are selected such that they have
different productivity and chain transfer agent responses. In other
words, under substantially the same conditions, the catalysts will
react differently to a temperature change and/or the addition of a
chain transfer agent into the system.
Due to the intrinsic molecular weight differences in the polymer
produced by the chosen high and low molecular weight catalyst, the
polymer produced by the two catalysts in a single reactor has a
high molecular weight fraction and a low molecular weight fraction.
Such a phenomenon is referred to herein after as "polymer split." A
polymer split is defined as the weight fraction of the high
molecular weight polymer component in a polymer with such split.
The relative fraction of the high molecular weight component can be
measured by deconvoluting a gel permeation chromatography ("GPC")
peak. One characteristic of the process described herein is that
the polymer split can be varied from 0 to 100% by adjusting the
ratio of the high molecular weight catalyst to the low molecular
weight catalyst. Because any two catalysts can exhibit different
catalytic efficiency at a given set of polymerization process
conditions, the polymer split may not correspond directly to the
molar ratio of the two catalysts.
Due to the complex dependence of melt/flow properties on the
position and shape of a polymer molecular weight distribution, the
method to make a polymer with a target melt flow index and melt
flow rate requires more than one variable. Based on the
productivity of the catalyst and the reaction to a chain transfer
agent, temperature and amount of chain transfer agent are used in a
coordinated scheme to target the melt index and melt flow ratio of
resin produced with the catalyst system.
The catalyst compounds which may be utilized in the catalyst
compositions of the invention include invention include: Group 15
containing metal compounds; metallocene compounds; phenoxide
catalyst compounds; additionally discovered catalyst compounds; and
conventional-type transition metal catalysts. Several suitable
catalysts and methods for preparing catalysts are described in U.S.
Pat. No. 6,846,886, which is herein incorporated by reference to
the extent that definitions therein do not conflict with the stated
or implied definitions presented herein.
Any catalyst system in which the two (or more) catalyst components
have substantially different chain transfer agent responses may be
used. The catalyst system can be a Ziegler-Natta catalyst combined
with a single site catalyst, two Ziegler-Natta catalysts, or two
single site catalysts. In a preferred embodiment, the catalyst
system is made up of two Ziegler-Natta catalysts. In a preferred
embodiment, the Ziegler-Natta catalysts have titanium and hafnium
active catalyst sites.
U.S. Pat. Application Pub. No. US2005/0228138A1 to Davis et al.,
which is herein incorporated by reference to the extent that
definitions therein do not conflict with the stated or implied
definitions presented herein, discloses several bicomponent
catalyst systems which may be implemented in various embodiments of
the present invention.
The use of polyselective catalysts is not limited to the catalysts
described in the above enumerated publications, which does not
represent an exhaustive list of such known olefin polymerization
catalysts. As a method for targeting the composite product of a
catalyst system is described, two or more polyselective catalysts
are used. A biselective catalyst is one which has two different
types of polymerization catalyst species in the same catalyst
composition; a polyselective catalyst is one which has two or more
different types of polymerization species in the same catalyst
composition. Most often, this means that two species are present on
the same support. Less frequently, the support itself acts as one
of the active catalyst species, and supports a different catalyst
species. In either case, since the two species are present in the
same composition, and polymerizes the olefin(s) simultaneously,
there is little or no chance that resin particles are made
including only one mode of resin product. In a preferred
embodiment, the weight ratio of the first catalyst component to the
second catalyst component remains substantially the same during the
polymerization process.
For controlling bimodal molecular weights, a method employing two
mixed (biselective) catalyst compositions may be used. For
instance, if one biselective catalyst blend independently generates
a product with a 70% HMW, 30% LMW split and the other generates a
50% HMW, 50% LMW product, the range of products possible for all
relative catalyst feed rates would be from 50 to 70% HMW, compared
to a range of 0 to 100% if separate HMW and LMW producing feeds are
used. This restriction in the range of possible products
significantly reduces the sensitivity of the overall system to
perturbations in relative catalyst feed flow rates. In a preferred
embodiment, the polymer comprises no more than 50 wt % of the HMW
fraction. In an alternate embodiment, the polymer comprises no more
than 30 wt % of the HMW fraction. In an alternate embodiment, the
polymer comprises no more than 10 wt % of the HMW fraction.
The feed rates of catalyst compositions A and B can be manipulated
in response to continuous or intermittent measurements, or a
process model, of the desired product property or properties. The
ratio of catalyst species X to catalyst species Y in a given
biselective catalyst composition can be selected to provide a
specific ratio of product having the property, or value thereof, of
interest under a known set of polymerization conditions. The
catalyst composition can then be referred to as one which provides
a predetermined content, or "split," of, for example, high
molecular weights compared to the overall product, which can differ
from the weight or molar ratio of the metal components of the
catalyst composition. In a preferred embodiment, the feed rates of
catalyst compositions A and B remain substantially the same during
steady state polymerization. In other words, the weight ratio of
the first catalyst to the second catalyst remains substantially the
same during the polymerization process.
In principle, any two biselective or other polyselective catalyst
compositions can be used, so long as they have an acceptable degree
of effectiveness in imparting the property or properties desired.
Typically they are bimetallic or polymetallic, but they can be
biselective or polyselective for reasons other than the type of
metal polymerization site. For example, the catalyst components can
respond to different promoters or modifiers, and/or they can
respond to chain terminators such as hydrogen in different ways or
in different degrees.
Some bimetallic catalysts employed in some approaches contain at
least two transition metals, one in the form of a metallocene and
one transition metal in the form of a non-metallocene, and have an
activity of at least about 1000 g polymer/g catalyst or about 50 kg
polymer/g of each transition metal. The bimetallic catalysts are
typically free of water.
Because of the different chain transfer agent response of each of
the two sources of transition metals in the bimetallic catalyst,
each produces a different molecular weight component under
identical olefin polymerization conditions. In some embodiments,
the metal of highest hydrogen response is present in amounts of
about 0.1 to about 0.8 weight percent; in preferred embodiments
that metal is hafnium. The metal of lowest hydrogen response may be
present in amounts of about 0.5 to about 3.0 weight percent; in
preferred embodiments that metal is titanium. This catalyst system
is catalytically effective to produce bimodal molecular weight
distribution product containing about 0.05 to about 0.95 weight
percent of the high molecular weight component. In a typical
product of this catalyst, about 20% of the polyethylene is produced
by the hafnium sites, and about 80% is produced by the titanium
sites.
In bimodal molecular weight distribution products, the weight
fraction of the HMW component should be in the range of about 0.05
to about 0.95, more preferably from about 0.10 to about 0.90 for
applications requiring broad molecular weight distribution resins.
The flow index (FI) of the bimodal molecular weight product should
be in the range of 2 to 100. If the bimodal molecular weight
distribution product has an FI of less than 2, the FI is too low
for processability. On the other hand, if overall polymer FI is too
high, then product toughness properties decrease. Hence, it is
necessary to control polymer FI in the polymerization reactor.
Product melt flow ratio (MFR) values are preferably in the range of
about 30 to about 250. Smaller MFR values indicate relatively
narrow molecular weight distribution polymers.
Chain Transfer Agents
In each of the aforementioned generally preferred approaches and/or
embodiments, a chain transfer agent is present in the reactor.
Chain transfer agents or telogens are used to control the melt flow
index in a polymerization process. Chain transfer involves the
termination of growing polymer chains, thus limiting the ultimate
molecular weight of the polymer material. Chain transfer agents are
typically hydrogen atom donors that react with a growing polymer
chain and stop the polymerization reaction of said chain. These
agents can be of many different types, from saturated hydrocarbons
or unsaturated hydrocarbons to aldehydes, ketones or alcohols,
including hydrogen and aluminum alkyls. By controlling the
concentration of the selected chain transfer agent, one can control
the length of polymer chains, and, hence, the weight average
molecular weight, M.sub.w. The melt flow index (I.sub.2) of a
polymer, which is related to M.sub.w, may be controlled in the same
way.
After the donation of a hydrogen atom, the chain transfer agent can
react with the monomers, or with already formed oligomers or
polymers, to start a new polymer chain. This means that any
functional groups present in chain transfer agents, for instance,
carbonyl groups of aldehydes and ketones, are introduced in the
polymer chains.
A large number of chain transfer agents, for example, propylene and
1-butene which have an olefinically unsaturated bond, can also be
incorporated in the polymer chain themselves via a copolymerization
reaction. This generally leads to the formation of short chain
branching of respectively methyl and ethyl groups, which lowers the
density of the polymers obtained. The chain transfer agent can be
hydrogen, water, or carbon dioxide. In a preferred embodiment, the
chain transfer agent is hydrogen.
The amount of the chain transfer agent can range from 0.1 to
700,000 ppmv (based on reactor volume), preferably from 100,000 to
500,000 ppmv. When H.sub.2 is employed as the chain transfer agent,
the hydrogen/ethylene concentration ratio ranges from 0.000001 to
3; preferably 0.0001 to 2 in a gas-phase fluidized bed reactor.
Although the chain transfer agent can be added separately, it can
also be added as a mixture, a cofeed with ethylene, etc. The
presence of the chain transfer agent acts to increase FI. The
increase in FI depends on the amount of chain transfer agent
employed and the composition of the catalyst system. Increases in
FI can range from 10 to 2000%, preferably 20 to 500% over that of a
resin produced in the absence of the chain transfer agent.
Operating Conditions
Except where defined herein, the operating conditions of the
reactor and other systems are not narrowly critical to the
invention. While general operating conditions have been provided
above for fluidized bed polymerization reactor systems, fluidized
and nonfluidized bed systems can, in addition to those listed
above, have widely varying process conditions, such as temperature,
pressure, fluid flowrate, etc.
Polymerization conditions generally refer to temperature, pressure,
monomer content (including comonomer concentration), catalyst
concentration, cocatalyst concentration, activator concentration,
etc., that influence the molecular weight of the polymer
produced.
A particularly desirable method for producing polyethylene polymers
is in a fluidized bed reactor system, such as, but not limited to,
one of the general systems described above. The molecular weight of
the polymer can be controlled by increasing or decreasing the
concentration of the chain transfer agent. Reactor temperature,
while affecting the average molecular weight of both the LMW and
HMW components, is effective primarily by changing the split of the
resin product. A different flow index and melt flow ratio is a
consequence of this split change, with the flow index typically
decreasing and the MFR typically increasing with an increase in HMW
component split. With a Ziegler Natta catalyst of hafnium and
titanium, the apparent catalyst productivities of the titanium and
hafnium catalyst components differ such that an increase in reactor
temperature increases the productivity of the LMW Ti catalyst
component much more than that of the hafnium catalyst component,
thus decreasing the resin split in products made at higher
temperatures.
Temperature is also an effective variable for controlling MFR.
Increasing temperature, which, in turn, decreases product MFR, also
hampers productivity. This inhibiting effect of temperature is seen
in pipe and film production as well as in blow molding. However, in
some approaches, the inhibitive effect of higher temperatures is
acceptable where the goal is to achieve a particular MFR. In one
approach, the temperature of the process ranges from about 30 to
about 130.degree. C., more preferably from about 75 to about
130.degree. C., and even more preferably from about 95 to about
110.degree. C.
When a fluidized bed reactor system is employed, the chain transfer
agent can be introduced separately from other components, e.g., via
independent feed line; or introduced therewith. The chain transfer
agent is preferably added continuously to the reactor.
Conventional wisdom was that increasing the concentration of chain
transfer agent in the reactor reduces polymer chain length, and
thus, the overall MW. Lower molecular weights in turn result in
higher flow indexes (polymers with shorter chain lengths are easier
to extrude than polymers with longer chain lengths). Consequently,
it was believed that reducing hydrogen concentration in the reactor
running a bimodal catalyst system would increase product MFR by
increasing the spread of the HMW and LMW product components. This
may be due to several factors. For example, in some processes
performed in the presence of a bicomponent catalyst, the chain
transfer agent predominantly decreases the average molecular weight
of the LMW component while having a less significant effect on the
HMW component. An increase in chain transfer agent decreases the
weight fraction of the HMW component to some degree, causing the
relative weight fraction of the LMW component to increase
(decreasing the split). In other processes, reducing hydrogen
during fabrication of materials with a HMW split near 50% or
greater increases MFR because polymer properties seem more closely
linked with HMW component. The spread might actually decrease with
a hydrogen reduction as the MFR goes up. In either case, the
decrease in HMW component also typically increases the overall FI
of broad or bimodal molecular weight distribution resin.
What has surprisingly been discovered is that at least for certain
polyolefins, especially polyolefins suitable for blow molding
applications, the MFR relationships with product FI and split are,
under some conditions, opposite those previously observed.
Particularly, an alternate regime has been discovered whereby, if
the chain transfer agent concentration is sufficiently reduced, the
MFR and spread trends with chain transfer agent concentration will
reverse, leading to the surprising conclusion that, upon reaching a
certain split, MFR actually decreases with decreasing chain
transfer agent. In other words, when the chain transfer agent is
reduced to a certain point, the MFR decreases instead of
increases.
Without wishing to be bound by any theory, the cause for this
result seems to be related to the change in dominance from the HMW
to the LMW component once the chain transfer agent concentration is
reduced to a certain level. Particularly, the unusual trend is
believed to result from a large difference in component split. One
way to understand the difference is to imagine a central split
level where MFR is highest. For some common products, the product
split is higher than this center level, so decreasing split (which
happens if FI is normalized while hydrogen concentration is
decreased) will move the split toward the maximum-MFR level, thus
increasing MFR. Alternately, e.g., with some blow molding products,
where the product split is below the maximum-MFR level. Decreasing
hydrogen concentration, e.g., in blow molding polymer production,
also causes the split to decrease, but now the split is moving
further from the maximum-MFR level, causing the MFR to
decrease.
One benefit of this surprising discovery includes allowing
flexibility in tuning bimodal blow molding products to make widely
varying MFRs at a high flow index and a great range of component
splits. For example, the surprising discovery makes creation of
materials having a target FI but lower MFR possible, where
conventional wisdom was that such materials were not possible.
In one general approach, the concentration of the chain transfer
agent in the reactor is altered to move the HMW and LMW peaks in
the same direction. Such altering may include increasing or
decreasing the concentration of chain transfer agent. The altering
also causes the MFR to move in a first direction. At least one
condition in the reactor is adjusted to achieve a MW split where
the MFR trend reverses direction. Such condition(s) that can be
adjusted to change the MW split may include one or more of
temperature, pressure, monomer content (including comonomer
concentration), catalyst concentration, cocatalyst concentration,
activator concentration, etc.
In a particularly preferred approach, the MW split is adjusted by
changing relative amounts of the first and second catalyst
components in the reactor. The MW split may be adjusted by
selectively poisoning one of the catalyst components more than the
other catalyst component. The MW split may also be adjusted by
selectively activating one of the catalyst components more than the
other catalyst component, e.g., by using water to increase
productivity of one of the catalyst components. Combinations of the
foregoing may also be used.
The altering to move the HMW and LMW peaks in the same direction
and adjusting the at least one reactor condition may be performed
concurrently, but need not be.
In one preferred approach, the FI of the polymer is maintained at
about a target level throughout the process of effectuating the
reversal of the MFR trend. The FI of the polymer may be maintained
by changing relative amounts of the first and second catalyst
components in the reactor. This may be done by adding additional
catalyst, changing catalysts, reducing one catalyst component, etc.
In one example, when the concentration of the chain transfer agent
is decreased, the FI drops. To raise the FI back up, a third
catalyst component (which may be the same as the first or second
catalyst component, or different therefrom) may be added to affect
the split by increasing either the HMW or LMW component of the
polymer. The split in turn affects the FI.
By changing the concentration of the chain transfer agent while
keeping the FI at about a target level, the overall viscosity of
the polymer can be maintained at about a constant level. However,
because of the reversal of the MFR trend, the shear thinning
property of the polymer (as determined by the MFR) can be tuned in
a new way. Because the polymers produced are typically
non-Newtonian, the shear viscosity drops with increasing rates of
extrusion. This phenomenon is known as shear thinning.
In yet another approach, the overall product FI is changed at least
once in the process.
In another general approach to making a polyolefin, one or more
olefinic monomers are contacted in a single reactor containing a
bicomponent catalyst system comprising a first catalyst component
and a second catalyst component each having a catalyst productivity
and a chain transfer agent response; the single reactor being
having a chain transfer agent concentration; the chain transfer
agent response of the second catalyst component having a different
sensitivity than that of the first catalyst component. The
polymerization of the olefinic monomers is effectuated to produce
an olefin polymer having a MFR and a FI, where the first catalyst
component produces a HMW fraction of the polymer, and the second
catalyst component produces a LMW fraction of the polymer. Reducing
the concentration of the chain transfer agent present in the
reactor while maintaining about a constant flow index causes the
MFR to decrease.
To assist the reader in conceptualizing the foregoing, consider the
following example. Assume a blow molding product is being produced
in a gas phase fluidized bed polymerization reaction in the
presence of a bimodal catalyst. The resin has a high split, and the
effect of the chain transfer agent on the process is as
conventionally observed. At initial steady state conditions, the
resin produced has a FI of "A" and a given split. Assume that next,
the chain transfer agent concentration is reduced by a certain
amount. This causes the FI to drop below level A. To bring the FI
back to level A, more LMW catalyst component is added to the
process to increase the LMW portion of the resin. This in turn
lowers the split while increasing the MFR. Assume that next, the
concentration of chain transfer agent is further reduced. The FI
drops more, so more LMW catalyst component is introduced into the
system to increase the FI back to level A. Consequently, the split
falls further, and the MFR increases again. Assume that next, the
concentration of the chain transfer agent is further reduced. Now
the MW peaks of the HMW and LMW components of the polymer product
are much higher than they were before. As before, more LMW catalyst
component is introduced into the system to increase the FI back to
level A. As expected the split also falls further. However, the MFR
trend reverses and begins to drop, instead of increasing as
expected. Thus, when the split reaches a certain point for a given
FI, the property relationships of polymer product change.
Those skilled in the art will appreciate that, at a given FI, and
using a bimodal catalyst where the two catalysts have different
response to a chain transfer agent, the particular split where the
MFR trend reverses will vary depending on various polymerization
conditions, the type of polymer being created, etc. However,
following the teachings set forth herein, those skilled in the art
should be readily able to recreate a polymerization where the MFR
trend reverses. The phrases, unless otherwise specified, "consists
essentially of" and "consisting essentially of" do not exclude the
presence of other steps, elements, or materials, whether or not,
specifically mentioned in this specification, as along as such
steps, elements, or materials, do not affect the basic and novel
characteristics of the invention, additionally, they do not exclude
impurities normally associated with the elements and materials
used.
For the sake of brevity, only certain ranges are explicitly
disclosed herein. However, ranges from any lower limit may be
combined with any upper limit to recite a range not explicitly
recited, as well as, ranges from any lower limit may be combined
with any other lower limit to recite a range not explicitly
recited, in the same way, ranges from any upper limit may be
combined with any other upper limit to recite a range not
explicitly recited. Additionally, within a range includes every
point or individual value between its end points even though not
explicitly recited. Thus, every point or individual value may serve
as its own lower or upper limit combined with any other point or
individual value or any other lower or upper limit, to recite a
range not explicitly recited.
All priority documents are herein fully incorporated by reference
for all jurisdictions in which such incorporation is permitted and
to the extent such disclosure is consistent with the description of
the present invention. Further, all documents and references cited
herein, including testing procedures, publications, patents,
journal articles, etc. are herein fully incorporated by reference
for all jurisdictions in which such incorporation is permitted and
to the extent such disclosure is consistent with the description of
the present invention.
While the invention has been described with respect to a number of
embodiments and examples, those skilled in the art, having benefit
of this disclosure, will appreciate that other embodiments can be
devised which do not depart from the scope and spirit of the
invention as disclosed herein.
EXAMPLES
It is to be understood that while the invention has been described
in conjunction with the specific embodiments thereof, the foregoing
description is intended to illustrate and not limit the scope of
the invention. Other aspects, advantages and modifications will be
apparent to those skilled in the art to which the invention
pertains.
Therefore, the following examples are put forth so as to provide
those skilled in the art with a complete disclosure and description
of how to make and use the compounds of the invention, and are not
intended to limit the scope of that which the inventors regard as
their invention.
The polymerization reactions described in the following examples
were conducted in a continuous pilot-scale gas phase fluidized bed
reactor of 14 inches internal diameter and about 4.6 to about 5.2
feet in bed height. The fluidized bed was made up of polymer
granules. The gaseous feed streams of ethylene and hydrogen
together with liquid comonomer were introduced below the reactor
bed into the recycle gas line. Hexene was used as comonomer in some
runs. The individual flow rates of ethylene, hydrogen and comonomer
were controlled to maintain fixed composition targets. The ethylene
concentration was controlled to maintain a constant ethylene
partial pressure of about 220 psia. The hydrogen was controlled to
maintain a constant hydrogen to ethylene mole ratio. Comonomer was
also controlled to maintain a constant comonomer to ethylene mole
ratio (of about 0.0007 for hexene). The concentrations of all the
gases were measured by an on-line gas chromatograph to ensure
relatively constant composition in the recycle gas stream.
The solid bimodal catalyst was injected directly into the fluidized
bed using purified nitrogen as a carrier. Its rate was adjusted to
maintain a constant production rate. The reacting bed of growing
polymer particles was maintained in a fluidized state by the
continuous flow of the make up feed and recycle gas through the
reaction zone. Superficial gas velocities of 1.9 to about 2.4
feet/sec was used to achieve this. The reactor was operated at a
total pressure of about 349 psig. The reactor was operated at
various reaction temperatures of 85-105.degree. C.
The fluidized bed was maintained at a constant height (about 4.6 to
about 5.2 feet) by withdrawing a portion of the bed at a rate equal
to the rate of formation of particulate product. The rate of
product formation (the polymer production rate) was in the range of
about 21.2 to about 49.4 lb/hour. The product was removed
semi-continuously via a series of valves into a fixed volume
chamber.
FIG. 4 is representative of the pilot-scale fluidized bed reactor
system used in these examples.
Example 1
Hydrogen Response
Hydrogen ratios tested in the runs varied greatly, ranging from
0.00075 to 0.003 hydrogen/ethylene ratios. The primary reason for
changing hydrogen concentration was to modify the product MFR, and
results with the blow molding product were unusual compared to pipe
and film products created with the same catalyst. Selected MFR
results at different hydrogen levels are shown in Table 1.
TABLE-US-00001 TABLE 1 MFR values at various H.sub.2/C.sub.2 gas
composition ratios at or near 30 dg/min flow index. H.sub.2/C.sub.2
FI MFR 0.00075 27.0 109 0.001 30.7 186 0.0015 28.8 240
As shown by the data in Table 1, there is a sensitive dependence of
MFR on hydrogen concentration while running the blow molding
catalyst. High hydrogen leads to high MFR, opposite the trend
typically seen in pipe or film production using the same
catalyst.
It was also observed that the MFR is highly sensitive to hydrogen.
Moving hydrogen from a 0.00075 hydrogen/ethylene molar gas ratio to
a 0.0008 can move MFR 20-40 units. This sensitivity suggests that
good hydrogen control will be critical for meeting tight MFR
specifications.
Example 2
MFR Values
MFR values from selected product parts are shown in Table 2.
Catalyst A is bis(2,3,4,5,6-pentamethylphenyl amido ethyl)anine
zirconium dibenzyl. Catalyst B is
bis(n-butylcyclopentadienyl)zirconium dichloride. Catalyst C is
(tetramethylcyclopentadienyl)(n-propylcyclopentadienyl)zirconium
dichloride. Such catalysts are available from Univation
Technologies, LLC (Houston, Tex.).
TABLE-US-00002 TABLE 2 MFR values for blow molding catalysts at
various reaction conditions. FI (I.sub.21 Run # Catalyst Trim
dg/min) MFR Density T.degree. (C.) H.sub.2/C.sub.2 1245-80 A/C C 39
320 0.958 85 0.003 1245-80 A/B B 28 180 0.956 105 0.0025 1245-90
A/C B 33 101 0.9567 105 0.00075 1245-120 A/C B 20.1 203 0.9577 100
0.0008 1245-120 A/C B 24.7 137 0.958 100 0.00076 1245-120 A/B B
21.2 160 0.9568 100 0.00075
Surprising and unexpected, from Table 2, it is seen that the MFR
decreases for a given catalyst as the hydrogen concentration
decreases.
* * * * *